Worm conveyors or vibration devices are usually employed for conveying a metered amount of bulk material.
To assure their operation, worm conveyors require a minimum gradient and a minimum diameter. With the components unchanged, it is possible to adjust the amounts to be conveyed up to a certain point by varying the rpm. In principle, however, with worm conveyors it is possible to achieve a satisfactory solution only with large-size bulk materials per unit of time.
For most bulk materials the lowest possible gradient is too high to assure, with vertical installation, the prevention of a run-off in respect to the storage bin. With embodiments with hollow spirals in particular there is no sealing.
Although a buildup of material (dirt) at the tube part of worm metering devices is prevented by the constant movement of the worm, the worm itself can easily experience an accumulation of material.
Vibrating metering devices permit very small metered amounts, but are very sensitive to jarring (regarding the unwanted emptying of the storage bin) and cannot be disposed vertically underneath a storage bin (require an inclined outlet surface). Furthermore, some bulk materials tend to form small lumps under vibration.